An electrochemical fuel cell is a device that reacts a fuel source with an oxidizing agent to produce an electric current. Commonly, the fuel source is a source of protons, such as hydrogen gas, and the oxidizing agent is oxygen. An example of a fuel cell utilizing these reactants is a proton exchange membrane (PEM) fuel cell, in which hydrogen gas is catalytically dissociated in the fuel cell's anode chamber into a pair of protons and electrons. The liberated protons are drawn through an electrolytic membrane into the fuel cell's cathode chamber. The electrons cannot pass through the membrane and instead must travel through an external circuit to reach the cathode chamber. In the cathode chamber, the protons and electrons react with oxygen to form water and heat. The net flow of electrons from the anode to the cathode chambers produces an electric current, which can be used to meet the electrical load being applied to the fuel cell by an associated electrical device, such as a vehicle, boat, generator, household, etc.
The fuel cell's ability to transport hydrogen ions across the membrane is a function of the hydration of the membrane. Preferably, the membrane is at or near saturation with water absorbed into the membrane, and this water conducts the hydrogen ions across the membrane. To achieve this desired level of saturation, the anode chamber is preferably at or near 100% relative humidity. However, at this level of humidity, water will tend to condense in the anode chamber. This water also must be periodically removed to prevent the operation of the fuel cell from being impaired. Too much water in the anode chamber will reduce the efficiency of the fuel cell because the water molecules will block the reacting sites of the anode and prevent hydrogen ions from reaching and being transported through the membrane.
In the cathode chamber, water is more prevalent because it is a byproduct of the reaction occurring at the cathode. In addition, water molecules are transported through the membrane with the protons, resulting in additional liquid water in the cathode chamber. When this flooding of the cathode chamber occurs and water droplets prevent oxygen molecules from reaching the cathode, the operation and efficiency of the fuel cell are impaired.
Therefore, there is a need to remove water from the chambers of the fuel cell. Typically, water is removed through periodic purging of either or both of the chambers through purge valves. These valves are briefly opened after a defined period of time elapses to depressurize the chamber. Accumulated water in the purged chamber is expelled with the gases in the chamber.
A problem with the conventional method of purging a fuel cell based on elapsed time is that the rate of water production is not proportional to the time elapsed since the fuel cell was last purged. For example, if the fuel cell is producing current at its maximum rate, it will produce and accumulate more water, and therefore require more frequent purging, than when producing current at a lower, or even nominal rate. Because the time interval at which the fuel cell is purged is fixed, sometimes the interval will be shorter than an optimum interval. Other times, it will be longer than the optimum interval.
Too infrequent purging of the fuel cell results in accumulation of water within the fuel cell, thereby producing the flooding and other undesirable conditions described above. Too frequent purging of the fuel cell will remove too much water, which will result in the drying of the membrane. As the membrane dries, its resistance increases, requiring more power to transport hydrogen ions across the membrane. This reduces the efficiency of the fuel cell. Another disadvantage of too frequent purging of the anode chamber is that hydrogen gas is exhausted when the anode chamber is purged. Since hydrogen is essentially the fuel required to produce current with the fuel cell, it can be understood that unnecessary purging of the anode chamber wastes fuel that could be otherwise used to produce an electric current in the fuel cell.
Neither of these conditions is desirable, so the fuel cell is conventionally purged based on a timed interval corresponding to an average rate of usage. For example, it may be purged for one-half of a second every thirty seconds of operation. As discussed, this purge cycle will be too infrequent for some operating states and too often for others.
Therefore, there is a need to optimize the purge cycle of a fuel cell, or fuel cell stack, based on the operating state of the fuel cell by correlating the purging of the fuel cell with the rate at which water is produced in the fuel cell. The invention described herein provides a system and method for optimizing the purge cycle of a fuel cell responsive to the performance of the fuel cell, thereby removing the problems encountered with too frequent or infrequent purging of the fuel cell. The system detects the value of a process parameter representative of the fuel cell's performance, and automatically actuates the fuel cell's purge assembly when this value exceeds a determined threshold value.
Many other features of the present invention will become manifest to those versed in the art upon making reference to the detailed description which follows and the accompanying sheets of drawings in which preferred embodiments incorporating the principles of this invention are disclosed as illustrative examples only.